Collectible receivables is the portion of what you are owed that you can realistically expect to receive.
It is always less than the total, and treating the total as money is one of the more common ways a business plans itself into trouble.
Why the total overstates it
Not every invoice gets paid. Some customers disappear, some dispute, some go under, and some invoices are simply too old to pursue with a straight face.
The likelihood of collection falls with age, sharply after the first two or three months. A receivable ledger with a long tail of very old invoices is not worth its face value and never was.
Reading it against the total
The gap between what you are owed and what is collectible is the honest measure of how well the business gets paid.
A narrow gap means invoices are chased and customers are good for it. A widening one means old invoices are accumulating rather than resolving, which usually shows up in days to get paid at the same time.
Where it matters most
Anywhere you are counting on receivables as cash. A runway calculated on the full receivable balance is longer than the real one, by exactly the amount you were never going to collect.
The practical response
Old invoices need a decision rather than continued hope. Chase it properly, settle it for less, or write it off — all three beat carrying it on the books as an asset while quietly knowing it is not one.
Writing off a genuinely dead invoice is not admitting defeat. It makes every number that depends on receivables true again.